1. Field of the Invention
The present invention relates to a pixel and an organic light emitting display device using the same.
2. Discussion of Related Art
In recent years, various flat panel display devices have been developed with reduced weight and volume as compared to cathode ray tubes. Types of flat panel display devices include liquid crystal display devices, field emission display devices, plasma display devices, and organic light emitting display devices, among others.
Of these flat panel display devices, the organic light emitting display device displays an image by using organic light emitting diodes, which generate light by recombining electrons and holes. Organic light emitting display devices are advantageous in that they have rapid response times and may be driven with relatively low power consumption.
FIG. 1 is a circuit illustrating a pixel of a conventional organic light emitting display device.
Referring to FIG. 1, the pixel 4 of the conventional organic light emitting display device includes an organic light emitting diode (OLED), and a pixel circuit 2 coupled to a data line (Dm) and a scan line (Sn) to control the organic light emitting diode (OLED).
An anode electrode of the organic light emitting diode (OLED) is coupled to the pixel circuit 2, and a cathode electrode of the organic light emitting diode (OLED) is coupled to a second power source (ELVSS). The organic light emitting diode (OLED) generates light with a luminance corresponding to an electric current supplied from the pixel circuit 2.
The pixel circuit 2 controls an amount of current supplied to the organic light emitting diode (OLED) in accordance with a data signal supplied to the data line (Dm) when a scan signal is supplied to the scan line (Sn). For this purpose, the pixel circuit 2 includes a second transistor (M2′) (e.g., a driving transistor) coupled between a first power source (ELVDD) and the organic light emitting diode (OLED); a first transistor (M1′) (e.g., a switching transistor) coupled between the gate electrode of the second transistor (M2′) and the data line (Dm), and having a gate electrode coupled to the scan line (Sn); and a storage capacitor (Cst) coupled between a gate electrode and a first electrode of the second transistor (M2′).
A gate electrode of the first transistor (M1′) is coupled to the scan line (Sn), and a first electrode of the first transistor (M1′) is coupled to the data line (Dm). A second electrode of the first transistor (M1′) is coupled to one terminal of the storage capacitor (Cst). Here, the first electrode of the first transistor (M1′) is either a source electrode or a drain electrode, and the second electrode of the first transistor (M1′) is the other of the source electrode and the drain electrode. For example, when the first electrode is the source electrode, the second electrode is the drain electrode. The first transistor (M1′) is turned on when a scan signal is supplied from the scan line (Sn), and supplies a data signal from the data line (Dm) to the storage capacitor (Cst). In this case, the storage capacitor (Cst) is charged with a voltage corresponding to the data signal.
A gate electrode of the second transistor (M2′) is coupled to one terminal of the storage capacitor (Cst), and a first electrode of the second transistor (M2′) is coupled to the other terminal of the storage capacitor (Cst) and the first power source (ELVDD). A second electrode of the second transistor (M2′) is coupled to an anode electrode of the organic light emitting diode (OLED). The second transistor (M2′) controls the amount of current in accordance with a voltage value stored in the storage capacitor (Cst), the current flowing from the first power source (ELVDD) to the second power source (ELVSS) via the organic light emitting diode (OLED). In this case, the organic light emitting diode (OLED) generates light in accordance with the amount of current supplied from the second transistor (M2′).
However, the pixel 4 of the conventional organic light emitting display device has difficulties displaying images with uniform luminance. More particularly, a threshold voltage of the second transistor (M2′) in each of the plurality of the pixels 4 may have different threshold voltage levels due to manufacturing process variances. When the threshold voltages of the drive transistors have different threshold voltage levels as described above, different luminances may be generated in the organic light emitting diodes (OLEDs), even though data signals corresponding to a same gray level are supplied to the plurality of the pixels 4.
Also, in a conventional organic light emitting display device, a voltage from the first power source (ELVDD) may be inconsistently applied due to voltage drops of the voltage from the first power source (ELVDD), depending on the positions of the pixels 4 in the display device. When the voltage from the first power source (ELVDD) varies according to the positions of the pixels 4 as described above, it is very difficult to display an image with a uniform or desired luminance.